This invention relates to refining of ore containing aluminum compounds. More particularly, this invention relates to an improved process for purifying spent liquor from a Bayer-type process prior to recycling the spent liquor back to the digest.
Ores containing aluminum compounds, more particularly bauxite type ores, are conventionally digested in caustic by the well-known Bayer Process to form a solution of sodium aluminate and sodium hydroxide. The solid bauxite residue (red mud) is then separated from the solution by clarification and, after washing to remove as much caustic content as economically feasible, is discarded. Alumina is recovered by precipitation from this solution by first cooling the solution to supersaturation and then seeding with aluminum trihydroxide. The spent mother liquor is then recycled back to the digester, after an evaporation step, for reuse of the caustic therein and to preserve the alumina values remaining in the spent liquor after precipitation.
Unfortunately, however, such ores may contain or form many other soluble compounds, such as sulfates, fluorides and carbonates which, due to their solubility, are not removed in the clarification step. These unwanted impurities can build up in concentration with repeated recycling of the spent liquor which can retard the recovery of alumina from solution, deposit scale on equipment (particularly heaters), reduce evaporation efficiencies, increase the required plant size (for given throughput) and operating costs due to the circulating liquor loads and increase the impurity level in the product. It would, therefore, be desirable to efficiently separate out such impurities from the Bayer liquor.
Malito et al U.S. Pat. No. 4,430,310 proposes a process for removing impurities from Bayer process using an organic solvent, such as methanol, methanol-isopropanol, and methanol-butanol mixtures. However, since these alcohols do not separate from the spent liquor into a separate phase, subsequent separation of the organic solvent from the spent liquor must be accomplished, such as by evaporation.
The yield of alumina in the precipitation step is further influenced by the ratio of alumina to caustic in the liquor. As the caustic level rises, due to depletion of alumina by precipitation, further precipitation of alumina is inhibited. This is sometimes overcome by neutralization of the sodium hydroxide, e.g., with sodium bicarbonate. However, this requires further reprocessing of the neutralized liquor to recover the soda for reuse in the digester.
If the caustic in the spent liquor could be selectively removed after the initial precipitation of alumina, the spent liquor stream could be reseeded and further alumina values could be precipitated from the spent liquor, prior to further treatment of the spent liquor to remove impurities.
Halff et al U.S. Pat. No. 3,511,606 discloses a process for treating solutions containing sodium hydroxide, such as Bayer Process liquors, with ethanol to extract the sodium hydroxide after separating out the aluminate sludge.
Aluminum-bearing ores, such as bauxite, also contain organic materials which are a composite of many substances described by such terms as humic, lignin, cellulose and protein. In caustic digestion processing, commonly known as Bayer processing, a portion of this organic matter is extracted into the liquor during digestion and is decomposed to form soluble sodium organic compounds. One of these degradation products has been identified as sodium oxalate. With recycling (for economic reasons) of the liquor after precipitation of aluminum hydroxide, the sodium oxalate concentration increases until an equilibrium concentration is reached.
Although it is known that pure sodium oxalate in the usual concentration of Bayer spent liquor, i.e., 180 grams/liter total caustic (TC), has little effect on the precipitation of aluminum hydroxide, plant oxalate, that is, oxalate which accumulates in the ore refining process and which is defined to include degraded organic materials, has been blamed for inhibiting precipitation of aluminum hydroxide. When this plant sodium oxalate concentration builds up to a critical supersaturated concentration, then it coprecipitates with the aluminum hydroxide giving rise to aluminum hydroxide fines and interfering with agglomeration of the aluminum hydroxide fines in the precipitation process. This causes serious problems in separating the fine aluminum hydroxide crystals from the mother liquor, resulting in more fine aluminum hydroxide being recycled back to digestion thereby decreasing the overall aluminum hydroxide yield. In order to obtain aluminum hydroxide product, having large particles formed, at least in part, by means of agglomeration, it is necessary to remove at least a portion of the sodium oxalate from the liquor stream. This reduces the accumulation of sodium oxalate and depresses its coprecipitation with aluminum hydroxide crystals in solution.
Several methods have been proposed to reduce the sodium oxalate concentration in caustic liquor. Byrns U.S. Pat. No. 3,337,305 proposes addition of ammonia to a caustic liquor containing oxalates to cause a precipitation of sodium oxalate salts. Yamada et al U.S. Pat. No. 3,899,571 provides for the removal of sodium oxalate by the addition of sodium oxalate seed crystals to a spent liquor to precipitate organic substances, mainly composed of sodium oxalate, which are then removed from the spent liquor.
Breteque U.S. Pat. No. 3,457,032 discloses a process for the purification of sodium aluminate obtained by alkaline digestion of bauxite which comprises treating the solution with strongly basic anion exchange resins of a macroreticular type to eliminate metal ion and organic acid impurities. Lever U.S. Pat. No. 4,275,042 describes the removal of sodium oxalate from Bayer spent liquor by treating the liquor with a cationic sequestrant which interacts with the humic material in the spent liquor to remove it and thereby destabilize the solution with respect to precipitation of sodium oxalate. The patentees also used seed crystals to expedite the precipitation of the sodium oxalate after the removal of the humic material. Gnyra U.S. Pat. No. 4,275,043 describes the removal of oxalate, generally as a disodium salt, from Bayer process liquor by treating the liquor with an adsorbent, such as activated carbon, to remove sufficient of the humic matter in the liquor to destabilize the sodium oxalate and cause it to precipitate.
Sato et al U.S. Pat. No. 3,649,185 describes a process for the removal of sodium oxalate from a spent liquor by increasing the caustic concentration of the liquor as well as controlling the temperature to develop supersaturation. The patentees further point out that the crystallized sodium oxalate is contained mainly in the finer part of the crystals and that this is used for further separation of the sodium oxalate.
In Bush U.S. patent application, one of us found that ethanol could be used to initially extract sufficient caustic from a Bayer spent liquor to permit a further precipitation of alumina following which the resulting spent liquor could be purified to remove sulfate and fluoride salts by further extraction of caustic with ethanol.
Subsequently we found in Bush et al U.S. patent application Ser. No. 587,589 that if 50 volume % or less of ethanol was used in an initial treatment of Bayer spent liquor, a sodium oxalate precipitate could be formed while the alumina values stayed in solution.